This invention relates to a DC motor such as a starting motor for an engine or the like, and more specifically, relates to a motor with U-shaped rotor-coil wires, each having two parallel leg portions and an intermediate portion inclined to a rotor-core end.
FIG. 1 shows a typical conventional DC motor in which a rotor 20 is rotatably disposed in a motor case 22. The rotor 20 consists of a rotation shaft 24, a cylindrical rotor core 30, and a rotor coil 36. The opposite end portions of the rotation shaft 24 is supported via bearings 32 and 34 by the opposite end brackets 26 and 28 of the motor case 22. The rotor core 30 is coaxially disposed and fixed onto the shaft 24 so that the opposite end faces 38 and 40 of the core 30 are opposed to the brackets 26 and 28, respectively.
As shown in FIG. 2, a plurality of slots 42 are formed in the peripheral face of the rotor core 30 in a conventional manner. An axially extending tooth 44 is formed between any two adjacent slots 42 and 42. Every slot 42 is of a radially elongated cross section of the same size, and has two radially aligned sections, namely inner and outer sections 46 and 48. By utilizing these slots 42, the rotor coil 36 is wound around the core 30.
FIG. 3 shows the core 30 with the coil 36 wound therearound in the manner of double-layer wave winding. This rotor coil 36 consists of a plurality of substantially inflexible U-shaped lead wires 50. As shown in FIGS. 4 and 5 (FIG. 4 is a perspective view), each of the lead wires 50 includes two spaced parallel leg portions 52 and 54 of the same length L.sub.0 and an intermediate portion 56 twisted in a substantially spiral fashion and interconnecting the leg portions 52 and 54.
To form the coil 36, these wires 50 are inserted from the rear end face 40 of the core 36 into the slots 42 in such a manner that one of the leg portions of a wire 50, i.e., the leg portion 52 is received in the outer section 48 of a slot 42, and the other leg portion 54 of the same wire 50 is received in the inner section 46 of another slot 42 (i.e., the sixth slot in FIG. 3). As a result, the intermediate portions 56 of the wires 50, each straddling a proper number of teeth 44 (five teeth in FIG. 3), project from the rear end face 40 of the core 30 (see FIG. 1).
On the rear end face 40 of the core 36, as shown in FIG. 3, the intermediate portions 56 of the wires 50, superficially, are laid substantially side by side, however in fact, the intermediate portions 56 cross over and under the neighboring intermediate portions 56 as if they were twisted together. All the crossings of the intermediate portions 56 are not disposed over the slots 42 but some of the crossings are disposed closer to the center of the rear end face 40 than the slots 42. In other words, some of the crossings are disposed in a radially inner region of the rear end face 40, inside an imaginary circle 58 (see a phantom line in FIG. 2) defined by the bottoms or radially inner ends of the slots 42. This twisted arrangement of the intermediate portions 56 allows the bulk of the intermediate portions 56 to be distributed substantially evenly over the rear end face 40 from the circumference of the core 30 to the radially inner region (see FIG. 1). More specifically, the intermediate portions 56 occupy a relatively flat donut-shaped volume having equal and constant thickness instead of occupying a relatively hilly donut-shaped volume only over the slots 42. The intermediate portions 56 in this twisted arrangement, practically, project less from the rear end face 40 than the intermediate portions 56 of which the crossings are disposed only over the slots 42, and therefore help the overall axial length of the motor to be minimized. However, according to the conventional twisted arrangement, more of the radially inner region of the rear end face 40 tends to be occupied by the intermediate portions 56 as the number or density of the slots 42 increases or as the slots 42 become deeper without the diameter of the core 30 being increased. This tendency is inconvenient since intermediate portions disposed too close to the center of the rear end face 40 can cause an interference with a central projection 59 on the rear end bracket 28, the central projection 59 consisting of a boss 60, a shaft aligner 62, a stopper 64 and the like (see FIG. 1), the axial length of the central projection 59 being larger at its central portion than at its periphery. Even the overall length of the motor must be lengthened to assure an adequate space between the intermediate portions 56 and the central projection 59 and thereby avoid interference.